The present invention relates to fluid packaging.
The present invention particularly relates to packaging using thin-walled extrusion blow moulded plastics bottles for fluids such as milk, which require to be filled and closed in a resealable manner.
The invention also relates to resealable cap closures for use with plastics bottles or composite material cans, and more specifically to such closures which provide tamper evidence.
In the specification that follows problems of packaging milk are specifically addressed. However, it will be appreciated that other pourable fluids such as fruit juice present similar packaging problems. The present invention is, however, only concerned with fluids that are not required to be packed in a pressurised manner. Accordingly, the problems of packaging carbonated drinks are not addressed.
The present invention in one aspect is also specifically concerned with types of packaging where the weight of the container is an issue and therefore relates specifically to thin-walled blow moulded plastics bottles.
In another aspect, the invention is concerned with resealable cap closures that reveal when tampering has taken place
1. The Technical Background
Conventionally, milk has been packaged in cardboard, gable top packs, which are notoriously difficult to open and result in numerous consumer complaints about milk spillage and difficulty in pouring. The fibre carton was only suitable for packaging liquids up to a capacity of 1.5 liters.
In order to resolve these problems blow moulded plastics polyethylene bottles have been used. These bottles are provided with resealable caps. The resealable caps are normally injection-moulded items. Since weight is significant in the packaging of fluids such as milk, these caps must also be light in weight. A weight of 2 to 4 g is usually the maximum that can be tolerated.
There is also a fundamental problem in achieving a good seal between a blow moulded bottle neck and an injection moulded plastics cap. This is because the tolerance of the neck is of the order of 0.3 mm whereas the tolerance of an injection-moulded item such as the cap is 0.1 mm. This means that a proportion of caps will not seal tightly when fitted to their necks. For all designs of caps this results in difficulties of fitting on the production line and, for retailers and distributors, leakage problems. The ultimate consumer may also have difficulty in resealing the bottle or opening it in the first place if the cap is over-tight.
A number of designs of injection moulded caps have been developed in an attempt to address these problems. For example, in a cap design known as a valve seal or pliable seal closure, a plug is provided in the cap which pushes into the neck of the bottle and a multiple start thread is provided on the interior wall of the cap skirt. This type of cap provides a double seal. The plug provides the seal against the inner wall of the neck. The second seal is provided by means of an inwardly projecting ridge above the threads on the inner wall of the cap, which seals against the outer wall of the neck. A pliable pull away ring around the lower edge of the cap can provide tamper evidence for this type of cap. With a cap made of low density polyethylene, it is possible to prise off the cap with the ring attached so that this form of tamper evidence is not very secure.
Another design known as the induction heat seal closure (IHS) provides a foil insert seated into the base of the cap. On the production line the filled bottles with caps fitted are passed through an induction heater, which fuses the foil to the neck of the bottle. When the consumer unscrews the cap the neck of the bottle is still sealed by the foil. This foil seal is pulled off in a separate operation. Severing the seal results in small hairs of the plastics material being raised on the surface of the bottle neck which can inhibit a good seal being formed when the cap is replaced after initial opening. The setting of parameters for the bonding process using an induction heat seal closure is critical in order to achieve a bond which is weak enough to allow the consumer to be able to peel away the foil, yet strong enough to maintain a good primary seal with the container neck. Because the presence of the foil means that no plug can be provided, the susceptibility to leakage in the consumer's home is increased as the resealing of the cap is poor. The cap is also relatively expensive as the provision of the peelable foil insert can add as much as 20% to the cost of the container.
Another set of problems arises from the production line process of filling the bottles and sealing them. Since the maximum linear speed of milk is restricted by the speed at which the milk starts to froth, the rate of filling depends upon the size of the nozzle used to pour the milk into the bottles. The nozzle size is constrained by the dimensions of the neck. For a typical milk container this is 38 mm. Larger necks allow for quicker filling but present greater sealing problems and require larger caps.
In the present context the term blow moulding refers to extrusion blow moulding rather than injection stretch blow moulding. In many modern production lines, a blow moulding plant is adjacent the dairy. This allows the bottles to be formed, filled and sealed in a single continuous production process. The most complex stage in blow moulding is balancing each parison and controlling the material distribution. The parison is then inflated against the wall of a temperature regulated mould solidifying to assume the shape of the mould cavity. In one conventional design of blow moulding machine a block of moulds shuttles between an extrusion station and a blowing station. The number of die-heads provided is generally equal to the number of cavities in the block or some fraction thereof. These die-heads are fed by a head manifold that typically results in an imbalance in the delivery of plastics material to each of the resulting parisons. This process results in difficulties in forming consistently the neck-portion of thin walled containers, achieving at best tolerances of +/−0.3 mm with repeatable accuracy. To achieve good performance with valve seal closures, it is imperative to form a perfectly round neck-bore with a minimum amount of ovality in both bore and threaded portion. Two processes are known to achieve the above result in multi-cavity blow moulding. They are namely a “pull-up” process, which is the lifting of a blow pin through a shear-steel assembly to cut a round bore in a bottle neck, or a “ram-down” process, which is the forcing downwards of a blow pin into a shear steel assembly. The drawback with pull-up is that the neck component is physically weak in its construction leading to poor sealing with valve seal closures as the bore relaxes over time causing leakage. Ram-down however, gives a very rigid neck but this has a weight disadvantage causing ovality of the neck coupled with added cost of material wastage. Ovality causes poor sealing with valve seal closures. Neither of these two processes is suitable for moulding pour-lip features on bottle-necks. With the pull-up finish it is almost impossible to mould a pour-lip feature and with the ram-down finish, it requires significant amounts of extra material and is almost impossible to mould without significant ovality and imperfections in the bore.
The above processes described relate to moulding machinery manufactured by companies such as Uniloy, Techne and Bekum, for example.
An alternative type of machine made by companies such as Graham Engineering and Uniloy, which is particularly suitable for on-site blow moulding plants, uses a process which is commonly referred to as wheel blow moulding. Unlike the previous processes described, the wheel produces only one parison at a time extruded from a single die-head. The mould blocks are mounted on a rotary wheel structure and pass over the parison closing as the wheel rotates. A needle assembly pierces the parison and inflates the plastics until it solidifies against the wall of the temperature regulated moulds. Wheel blow moulding gives a high level of control in material distribution in containers produced in this way. The set up time for such a machine is significantly reduced, as only one die-head needs to be set up.
Where the inner wall of the neck provides one part of a seal, it may be necessary to provide a separate finishing station where the neck is either reamed or punch finished. The finishing step may produce swarf, which results in the risk that the swarf could enter inside the bottles and make them unsuitable for immediate filling.
For products such as milk where large quantities are required to be distributed through the retail chain, it is highly desirable to minimise the weight of the packaging. This has resulted in larger containers and thinner walls. Typical wall thicknesses for blow moulded high-density polyethylene (HDPE) are 0.4 to 0.6 mm. This results in a 4 pint (2.27 liters) bottle having a weight of around 40 g. Therefore any solution to the technical problems described must not increase the weight of the bottle and preferably would allow weight reduction.
2. Prior Art
For cardboard cartons it has been proposed to provide a separate spout assembly which is secured to the carton. An example is described in WO-A 96/14249 (Capitol Spouts Inc.). This spout includes a cap and an integral inner membrane seal and is assembled to an outer wall of a filled carton. The container may have a scored portion so that when the inner membrane seal is removed, it brings with it the scored portion of the container wall creating an opening through which the contents of the container can reach the spout. This assembly is not suitable for use with a plastics container where it would be impractical for the user to tear an opening in a plastics walled container. The cardboard carton will typically have a continuous inner lining. This type of spout must be fitted to the carton prior to filling and is not used for filling the container.
GB-A-2 108 464 (Container Corporation of America) describes an end closure arrangement wherein a membrane is sandwiched between and used to bond rim portions of a container body and end member to each other. The membrane has heat activatable sealing materials on both sides such as polyethylene, polypropylene or other similar types of material. The reader is told to use this type of closure with a container, which may be of all plastics, or a combination of paperboard and plastics materials. The exact method of production of the container body and end member is not further described. The specification is also silent as to the method of filling the resulting container. The specification particularly suggests use with a cylindrical cardboard container. Such containers would normally be filled from the base once the openable end had been completed and sealed.
U.S. Pat. No. 4,815,618 (Gach) shows a tamper indicating closure for a bottle designed for dry contents. A base section has a skirt, which engages with the neck of the bottle and defines a spout. A foil is interposed between the neck of the bottle and an adjacent surface of an upper part of the base. A pull ring is attached to a disc, which is connected to the opening in the upper part of the base by means of breakable webs. The disc is bonded to the foil. Pulling on the pull ring, which tears the foil away from the spout, opens the closure. In an alternative embodiment of the Gach invention, the disc is not joined to the base section and the foil is provided with a circumferential score line to facilitate tearing at the edge of the inner surface of the spout. In either embodiment a clean opening is unlikely to be produced. This would not be a problem when the bottle is used for tablets or the like but a torn foil edge within the spout is unsuitable for the pouring of liquids. The material of the bottle is not disclosed.
Although these documents are referred to as the most relevant prior art they do not represent a natural starting point for those seeking to solve the technical problems described in relation to thin-walled plastics bottles, in which the teaching has hitherto been directed exclusively at integral formation of the bottle body and neck.
Therefore, although it is known to produce a separate component defining a neck as in GB-A-2 108 464, the possibility of using this approach to solve the long present technical problems of effective reclosable sealing of thin-walled blow moulded plastics containers for fluids had not hitherto been appreciated and cannot therefore be regarded as obvious.